1. Field of the Invention
This invention relates generally to methods and apparatus for pulling a flowline to a subsea structure for making a connection between the flowline and the subsea structure.
2. Description of the Prior Art
Because an offshore oil production platform can be located at a remote distance from a well whose production is to be conveyed to the production platform or to which injection fluid is to be sent from the production platform, for example, it is necessary to extend a flowline through the sea between the well and the production platform. A flowline which can be used for this purpose and a method of towing the flowline into position between the well and the production platform are disclosed in my co-pending U.S. patent application Ser. No. 048,316 entitled Flow Line Bundle and Method of Towing Same, filed June 14, 1979.
Once the flowline, such as the one disclosed in my aformentioned patent application, is towed or otherwise placed in a position in the vicinity of the well and the production platform, the flowline must be connected at one end to the production platform and at the other end to the well so that the oil from the well can be conveyed to the production platform or injection fluid can be sent to the well from the production platform, for example. One method of making these connections is disclosed in my aforementioned co-pending application. This disclosed method calls for hauling one end of the flowline into a receiving module by means of a line extending downwardly from the surface structure of the production platform.
Other means for pulling ends of pipelines or the like into subsea structures have been disclosed. U.S. Pat. No. 4,075,862 to Ames discloses that downhaul cables are acoustically called up from subsea structures and attached to powered winches of two flowline mating vehicles attached to the ends of a flowline while the ends of the flowline are located on two ships. The flowline is then dropped into the sea and hauled down to the subsea structure by the powered winches. Umbilical power cords from the two winches are connected to the flowline mating vessels.
Pull-in cables attached to platform-mounted or buoyant winches are disclosed in each of the following publications:
Ocean Resources Engineering, April 1978, "Laying Underwater Pipeline by Float and Chains Method";
Paper OIC, 3074, "Second End Flowline Connection Without Length Adjustment," by S. Pras, 10th Annual OTC, 1978;
Offshore Engineer, September 1978, pp. 28-31, "Murchison Gave Conoco Team Chance For Subsea System Innovations";
Petroleum Engineer International, January 1979, pp. 28, 30, 32 and 34, "Satellite Wells Completed"; and
U.S. Pat. No. 4,145,909 to Daughtry.
The pulling-in of ends of pipelines by buoyant winches is shown in U.S. Pat. No. 3,530,680 to Gardner and U.S. Pat. No. 3,698,348 to Morgan.
Because of the translations and rotations which a flow-line goes through in a subsea environment, pull-in cables used to connect the flowline with a subsea structure can become entangled. Additionally, heave compensation is necessary when pull-in cables from a surface vessel are used because of the surface movement of the sea. Therefore, there is the need for an apparatus and method which can be used with a flowline and which can pull the flowline into the subsea structure without being affected by surface conditions and entanglement of lines and without requiring heave compensation apparatus.